It is common in the manufacture of lightweight metal fasteners to produce the fasteners in an automated process, whereby hundreds of articles are formed each minute. In order to maintain a high degree of quality control, it is important to include a step in the manufacturing process whereby a high volume stream of fasteners may be easily examined for conformity with established production standards.
Typically, a plurality of fasteners are advanced along a manufacturing line by means of physically engaging the extending flanges of a headed portion in a gripping mechanism and sequentially feeding the fasteners through an examination station. A common way of efficiently feeding a fixed number of fasteners, say 10 or 20, in consecutive batches through an examination station, is to load a batch of fasteners into a carriage/carrier which is then incrementally indexed through the examination station, whereby each fastener is in turn subjected to examination. The fasteners are supported within the carriage by means of engagement of the head of the fastener with an adapted receiving surface. After a fastener is examined, it is subsequently discharged from the carriage into either a `reject` receptacle or, if conforming, into an `accept` receptacle. A supply of fasteners is continuously directed into the vacancies established in the carriage by the discharge of examined fasteners.
In order to insure an acceptable level of material integrity within the fastener, it is becoming popular to subject metal fasteners to eddy current examination, whereby a high frequency current is induced into the surface of the fastener. Analysis of the resulting signal provides information representative of the existence of cracks in the fastener, material composition, and the presence of various surface treatments.
A significant problem with eddy current examination of fasteners is that due to the manner in which the fastener is advanced through the examination station, namely, by being suspended from a carriage with the underside of the head resting on a support surface, it is not possible to induce eddy current into the area of the fastener at the interface of the shank and the head. Unfortunately, this shank-head interface is the area of the fastener where material defects most often arise in the manufacturing process. Thus, the portion of the fastener which is most critical to establishing the acceptability of the article is inaccessible to eddy current examination. Furthermore, a fully comprehensive eddy current examination requires that the entire periphery of the fastener be subjected to the electromagnetic field of the differential eddy current probe. Advancing the fastener along a feed path in an orientation fixed relative to that of the eddy current probe allows the examination of only a fraction of the critical head-shank interface.
An additional problem exists with respect to damage to the eddy current probe. In order to optimize the scanning capability of the probe it is important to position the head of the probe as closely as possible to the fastener being examined. However, due to nonuniformity of the sizes of the heads, a fastener having an oversized head may come into contact with a probe positioned too closely to the examination station. This contact leads to costly damage to the eddy current scanning equipment as well as to the downtime associated with a crippled quality inspection line.
No solution currently exists for solving the above identified problems. Current manufactures are faced only with the option of scanning areas away from the shank-head interface and attempting to infer from that measurement the condition of the fastener directly under the head. Additionally, eddy current probes must be spaced from the examination a distance sufficient for improperly formed heads to avoid contact with the probes.